88 MOLLUSCA. 



PLACUNA, Brug. 



A small genus allied to the Anomiae, in which the valves are thin, 

 unequal, and frequently irregular, as in the latter, but both entire. 

 Two projecting ribs, en chevron, are seen on the inside of one of 

 them, near the hinge. 



The animal is not known, but it must resemble that of the Ostrese, 

 or that of the Anomiae *. 



SPONDYLUS, Lin. 



A rough and foliaceous shell as in the Ostreae, and frequently spiny ; 

 but the hinge is more complex ; besides the cavity for the ligament, 

 analogous to that of the Ostreae, there are two teeth to each valve 

 that enter into fossae in the opposite one; the two middle teeth be- 

 long to the most convex valve, which is usually the left one, and 

 which has a projecting heel, flattened as if sawed through behind 

 the hinge. The animal, like that of a Pecten, has the borders of its 

 mantle furnished with two rows of tentacula, some of the external 

 ones being terminated by coloured tubercles ; before the abdomen is 

 a vestige of a foot formed like a broad radiated disk on a short pe- 

 dicle, and endowed with the faculty of contraction and expansion f. 

 From its centre hangs a filament, terminated by an oval mass, the use 

 of which is unknown. 



The Spondyli are eaten like oysters. Their shells are frequently 

 tinged with the most brilliant colours. They adhere to all sorts of 

 bodies J. 



PLICA TULA, Lam. 



The Plicatulae, separated by Lamarck from the Spondyli, have 

 nearly the same kind of hinge but no heel, and flat, almost equal, irre- 

 gular, plicated and scaly valves, as in many of the Ostreae . 



MALLEUS, Lam. 



A simple pit for the ligament as in the Ostrese, where the Mallei were 

 left by Linnaeus, on account of their having the same irregular and 

 inequivalvc shell, but distinguished by a notch on the side of this liga- 

 ment for the passage of a byssus. 



The most known species, Ostrea malleus, L.; Chemn., VIII, 

 Ixx, 655, 656, which ranks among the number of high-priced 

 and rare shells, has the two ends of the hinge extended and 

 forming something like the head of a hammer, of which the 

 valves, elongated in a transverse direction, represent the handle. 

 It inhabits the Archipelago of India. 



There are some others, possibly young ones of the same species, in 



* Anomia placenta, Chemn., VIII, Ixxix, 716; An. sella, Ib., 714. See also 

 pi. 173 and 174, Encyc. Method., Vers. 



f Called by Poll " the abdominal trachea" in the Spondyli, &c. 



J Spondylus gteckropus, Chemn., VII, xliv, et seq., IX, cxv ; Sp. regius, Id., xlvi, 

 471. 



Spond. plicatus, L., Chemn. VII, xlvii, 479, 482; Plicat. efgyptia, Savign., 

 Fpyp. Coq. pi. xiv, f. 5. 



